Engines may utilize humidity sensors at various locations for controlling engine operation, such as for controlling spark timing and exhaust gas recirculation (EGR). Errors in the humidity reading can lead to improper spark timing, and thus knock, as well as various other issues with regard to condensate formation, EGR control, diluent control, etc.
Often, a diagnostic is run to check the functionality or rationality of an engine humidity sensor, and this diagnostic may use a comparison of the humidity sensor output with a humidity as estimated from signals from an exhaust gas oxygen sensor, pressure sensors, temperature sensors and the like, or with a humidity as measured using a second humidity sensor in another location in the engine system. Issues may arise with this technique if the humidity between the two sensor locations is different, as is often the case between an intake and an exhaust system during engine operation, or differences may arise due to other noise factors such as completeness of combustion, exhaust gas temperature, and the like. Additionally, for vehicles such as hybrids the engine may have limited operating time, and the conventional diagnostics may therefore have limited opportunities to be conducted. Furthermore, in arid climates with little or no humidity in the ambient air, it may be difficult to test or diagnose the sensor.